I reported my neighbours who claim full housing benefit and work, twice! I work for their landlords so I know they are on full housing benefit. What have they done, nothing! The first time I reported them on the anon number it never got to the right people. But still nothing has been done, but their garden looks good and they are always decorating!
I too was scared that they would guess who it was, but seeing as they seem to do nothing I shouldn't worry. Report them to ease your conscience, safe in the knowledge no one will look in to it!